Prior to the present invention various means were used to control the amount of plastic (shot size) injected into the mold during each cycle of an injection molding machine. For example, in the system shown in U.S. Pat. No. 3,767,339 issued to D. B. Hunkar the ram velocity is controlled as a function of the position of the ram in order to control the injection rate of plastic and thus the total volume of plastic injected during the cycle.
In U.S. Pat. No. 3,797,808 issued to W. Y. Ma, et al there is shown a system for controlling the shot size and plastication time in an injection molding machine. This system includes a computer for calculating screw speed based on previous cycle operation conditions in order to control a plastication time to a predetermined value. Additionally, the back pressure on the rotating screw is controlled to maintain the melt temperature of the plastic and thus the amount of plastic injected into the mold to a predetermined value.
In the machine of U.S. Pat. No. 3,932,083 issued to J. E. Boettner the position of the ram is monitored at a certain time after the start of the injection cycle and the injection pressure is adjusted, if needed, to compensate for the viscosity of the melt. After fill the mold pressure is monitored and additional pressure applied as needed to complete the fill.
The above noted control systems are much less accurate and efficient when used at high speed injection rates.
In another prior art system designed for high speed operation hydraulic oil is pumped into a piston-type accumulator against a volume of dry nitrogen pre-charged to a known pressure. For injection the hydraulic oil is released through a valve into the injection cylinder. The stroke of the injection piston or the injector rod is measured by a linear displacement transducer. When the desired stroke, i.e. injected volume, is reached the valve between the accumulator and the injector cylinder is automatically closed. As the speed of this system was increased its repeatability decreased. At very high injection rates the inaccuracies of the last described system produced unacceptable rejection rates.